1. Field of the Invention
The present invention relates generally to methods and apparatus for making molded plastic articles. In particular, the present invention relates to rotational molding methods and apparatus for making multi color plastic articles with controlled placement of differently colored plastic powders.
2. Description of the Related Art
Rotational molding is commonly used to produce hollow plastic articles. Rotational molding is unique among plastic molding processes because the heating, shaping and cooling of the plastic all take place inside the mold without application of pressure. The basic rotational molding process involves placing a predetermined charge of cold plastic powder into a cold mold, which is usually formed of sheet steel or cast aluminum. The mold is then closed and rotated biaxially in a heated oven. As the inner surfaces of the metal mold become hot, the plastic powder tumbling inside the mold starts to melt and coat the inner surface of the mold. When all the powder has melted, the mold is then transferred to a cooled environment. The biaxial rotation typically continues in the cooled environment until the plastic has solidified. At this point, the mold is opened and the molded article is removed.
A wide variety of shapes and sizes of articles can be rotationally molded. However, conventional rotational molding processes are not well suited for producing multi colored articles having uniform wall thickness with abrupt color changes. Such coloring is typically provided by time consuming and expensive post-molding coloring operations, such as painting, silk screening, and the like. Such coloring has also been provided by separately forming different colored sections of an article and then xe2x80x9cweldingxe2x80x9d or melting the different colored sections together. This is also a time consuming and expensive operation and often produces articles of relatively poor quality.
Thus, there is a need in the molded plastics industry for an improved rotational molding apparatus and method that produces nearly seamless multi color articles with abrupt color changes without requiring post-molding coloring operations.
It is an object of the present invention to provide an improved rotational molding method and apparatus that solve the problems with the conventional rotational molding methods and devices described above.
More specifically, the principal objects of the present invention are to provide a rotational molding method and apparatus that: use controlled placement of multiple pigmented plastic powders to produce multi color articles; produce essentially seamless multi color articles without requiring post-molding coloring operations; can produce open or box-shaped rotationally molded articles having differently colored sections; and produce multi color articles having smooth surfaces and abrupt color changes between differently colored sections.
It is a further object to provide a rotational molding apparatus that is economical to manufacture, efficient in use, capable of a long operating life, and particularly well suited for rotational molding of articles having differently colored sections.
In order to realize the objects and advantages set forth above, the Applicant has developed an improved rotational molding apparatus for rotational molding of plastic articles having differently colored sections. The method and apparatus use a rotational mold having first and second mold shells separated by a removable partition member to form first and second interior spaces within the mold. A thin blade is held against the inner surface of at least one of the mold shells to define a plurality of compartments within the respective interior spaces.
Differently colored molding materials, preferably plastic powders, are introduced into each of the compartments and caused to melt against the inner surfaces of the mold shells by heating and rotating the mold. The surfaces of the blade and the partition member are kept at a lower temperature than the inner surfaces of the mold shells to prevent the molding materials from melting against the surfaces of the blade and the partition member. The partition member and blade are then removed by separating the first and second mold shells, and additional molding material, such as non-colored, translucent plastic powder, is added to the mold. The rotational mold is then closed, and again heated and rotated to cause the additional molding material to melt against the colored molding materials on the inner surfaces of the mold shells. The result is a rotationally molded plastic article having controlled placement of differently colored molding materials and a nearly seamless interface between the differently colored sections.
According to a broad aspect of the present invention, a rotational molding method is provided, comprising the steps of: providing a mold having an interior space defined by an inner surface; separating the interior space into first and second compartments by engaging a blade against the inner surface; introducing a first molding material into the first compartment, the first molding material having a first color; introducing a second molding material into the second compartment, the second molding material having a second color different from the first color; heating and moving the mold to cause the first and second molding materials to melt against the inner surface separately from one another without adhering to the blade; removing the blade from the interior space; introducing a third molding material into the interior space; and heating and moving the mold to cause the third molding material to melt against the first and second molding materials melted against the inner surface.
According to another broad aspect of the present invention, a method of making multi color molded articles is provided, comprising the steps of: providing a mold having first and second separable mold portions each having an inner surface; placing a partition between the first and second separable mold portions to separate a first interior space defined by the inner surface of the first mold portion from a second interior space defined by the inner surface of the second mold portion; introducing a first molding material into the first interior space, the first molding material having a first color; introducing a second molding material into the second interior space, the second molding material having a second color different from the first color; heating and moving the mold to cause the first and second molding materials to melt against the inner surfaces of the first and second mold portions separately from one another without melting against the partition; removing the partition from between the first and second mold portions; introducing a third molding material into the interior spaces defined by the first and second mold portions; placing the first and second mold portions in close peripheral engagement with each other; and heating and moving the mold to cause the third molding material to melt against the first and second molding materials melted against the inner surfaces of the first and second mold portions.
According to yet another broad aspect of the present invention, a rotational molding apparatus is provided, comprising: first and second mold shells each having an inner surface; a partition member adapted to be clamped between the first and second mold shells to form a first interior space on a side of the first mold shell and a second interior space on a side of the second mold shell, the first and second interior spaces being separated from each other by the partition member; and an alignment arrangement for positioning the first and second mold shells in alignment with each other with the partition member therebetween, and for positioning the first and second mold shells in alignment with each other without the partition member therebetween, whereby the partition member can be selectively removed for different molding operations.
The molding apparatus further comprises at least one blade attached to the partition member. The blade is arranged to engage the inner surface of the first mold shell and divide the first interior space into a plurality of compartments for containing different colored molding materials. The blade has a thin edge in engagement with the inner surface of the first mold shell to minimize conductive heat transfer from the first mold shell into the blade and to define an abrupt change of color. The partition member and the blade may be provided with heat sinks to keep the respective surface temperatures of the partition member and the blade substantially below a temperature of the inner surface of the first mold shell.
Numerous other objects and advantages of the present invention will be apparent to those skilled in this art from the following description wherein there is shown and described a preferred embodiment of the present invention, simply by way of illustration of one of the modes best suited to carry out the invention. As will be realized, the invention is capable of other different embodiments, and its several details are capable of modification in various obvious aspects without departing from the invention. Accordingly, the drawings and description should be regarded as illustrative in nature and not restrictive.